Within a radio receiver, a phase detector is used to recover the phase information from the received signal. In a system utilizing a digital phase modulation scheme, a symbol slicer determines the symbol that is represented by the recovered phase information. Two methods of detecting the phase of a received signal are baseband I and Q processing and the direct phase quantization of a limited intermediate frequency (IF) signal.
One implementation of a phase detector operates in an I and Q digital system. In this system, the IF signal from the radio receiver is input into an I/Q translator which recovers the inphase (I) portion and the quadrature (Q) portion of the signal. The I and the Q signals are input into a pair of analog-to-digital converters (ADCs). The outputs of the ADCs are used to perform a tangent inverse of Q over I to produce a digitized phase signal. This system uses several individual components. Two high current ADCs, a ROM look-up table and a digital signal processor (DSP) are necessary to implement this system.
A second type of phase detector, which directly quantizes the phase of a limited IF waveform, is a completely digital implementation. Here, the modulated IF input signal is limited to a predetermined voltage range. This limited input signal is then compared to 2.sup.N-1 phase shifted reference signals at the intermediate frequency. The regenerated reference signal having a phase that most closely resembles the limited input signal is chosen. The selected reference signal is used by the phase processing circuitry to produce an N-bit digital word representing the phase of the IF input signal. Because this implementation requires the generation of 2.sup.N-1 reference signals, it is relatively complex.
In order to allow radio receivers to be implemented in smaller, simpler devices such as portable radiotelephones, there is a need for a low cost digital phase detector having a small size and low power consumption.